Don't pass in a vector to Encoder::new.

It's not necessary.

compiler/rustc_codegen_ssa/src/lib.rs

@@ -203,7 +203,7 @@ const RUSTC_VERSION: Option<&str> = option_env!("CFG_VERSION");
 
 impl CodegenResults {
     pub fn serialize_rlink(codegen_results: &CodegenResults) -> Vec<u8> {
-        let mut encoder = opaque::Encoder::new(vec![]);
+        let mut encoder = opaque::Encoder::new();
         encoder.emit_raw_bytes(RLINK_MAGIC).unwrap();
         // `emit_raw_bytes` is used to make sure that the version representation does not depend on
         // Encoder's inner representation of `u32`.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -2194,7 +2194,7 @@ pub fn encode_metadata(tcx: TyCtxt<'_>) -> EncodedMetadata {
 }
 
 fn encode_metadata_impl(tcx: TyCtxt<'_>) -> EncodedMetadata {
-    let mut encoder = opaque::Encoder::new(vec![]);
+    let mut encoder = opaque::Encoder::new();
     encoder.emit_raw_bytes(METADATA_HEADER).unwrap();
 
     // Will be filled with the root position after encoding everything.

compiler/rustc_serialize/src/opaque.rs

@@ -18,8 +18,8 @@ pub struct Encoder {
 }
 
 impl Encoder {
-    pub fn new(data: Vec<u8>) -> Encoder {
+    pub fn new() -> Encoder {
-        Encoder { data }
+        Encoder { data: vec![] }
     }
 
     pub fn into_inner(self) -> Vec<u8> {

compiler/rustc_serialize/tests/opaque.rs

@@ -31,7 +31,7 @@ struct Struct {
 fn check_round_trip<T: Encodable<Encoder> + for<'a> Decodable<Decoder<'a>> + PartialEq + Debug>(
     values: Vec<T>,
 ) {
-    let mut encoder = Encoder::new(Vec::new());
+    let mut encoder = Encoder::new();
 
     for value in &values {
         Encodable::encode(value, &mut encoder).unwrap();